This invention relates generally to the field of mass storage devices for use in digital data processing systems and more specifically to magneto-optical data storage devices.
Magneto-optical mass storage devices record data by using a focused laser beam to locally heat a minute portion of a magnetic storage medium, which is typically is constructed as a rotating disk. The coercivity of that portion of the medium is thereby lowered, allowing the magnetic polarity in that portion to be reversed by an applied magnetic field of lower intensity than would otherwise be required. Retrieval of data is accomplished by illuminating successive portions of the storage medium with a linearly polarized laser beam. The Kerr rotation effect causes the plane of polarization of the illuminating beam to be rotated clockwise or counterclockwise, depending on the magnetic polarity in the illuminated portion of the storage medium. This polarization rotation is sensed, typically with a pair of optical detectors and a polarizing beam splitter, to produce an output data signal.
Limitations on the efficiency of magneto-optical storage devices involve the data transfer rate, i.e. the rate at which data is written onto the storage medium, and the signal-to-noise ratio characterizing retrieval of data from the medium. A high data transfer rate requires high laser power at the medium during the write operation so as to rapidly heat successive portions of the medium in which the data is to be written. A high signal-to-noise ratio requires a high transmittance in the path of energy reflected from the medium to the detectors.
In typical prior magneto-optical storage devices these requirements conflict. A single laser is used for both writing and reading and a beam splitter is included in the optical train to direct a portion of the light reflected from the storage medium onto the optical detectors. A compromise must thus be reached between the amount of light that reaches the medium during the write operations and the amount that reaches the optical detectors during retrieval (read) operations.
Specifically, if the beam splitter is to have high transmittance, the write laser power will be high but the signal-to-noise ratio during read operations will be low. The converse will be true if the beam splitter is made to have a high reflectance. In a typical-magneto optical storage device the beam splitter has a transmittance of approximately 50%. As a result the portion of the laser output reaching the storage medium during write operations is somewhat less than that, typically only 30-40%.
Moreover, the write operations have typically required a data verification cycle following the writing of data onto the storage medium in order to make sure that the data has been accurately stored. This materially reduces the effective data transfer rate.
It should be noted that the foregoing problems are not inherent in write-once or read-only optical storage devices. In these devices data retrieval does not utilize the polarization properties of the laser beam. High efficiency is therefore achieved in both the read and write operations by the simple expedient of incorporating a quarter-wave plate into the optical path. The plate is positioned between the beam splitter, which is polarization sensitive, and an objective lens that converges the light beam impinging on the storage medium. This provides a highly efficient mechanism for separating the reflected light from the incident beam. In a magneto-optical device a quarterwave plate cannot be used, as it causes the medium to be illuminated by circularly polarized light, whereas linear polarization is needed in order to observe the Kerr rotation effect during data retrieval operations.